Airport and airline safety are at the forefront of current national and international concern. There are obvious concerns about the safety of airplanes and the potential for an electronic failure during a flight. This has been amplified of late due to the threat of terrorism and tampering with airplanes to produce failures. Other aircraft failures that are not terror related continue to demonstrate an ongoing need to continue the advance of airplane safety.
One such occurrence, the crash of Trans World Airlines (TWA) 800 on Jul. 17, 1996, prompted the creation of the White House Commission on Aviation Safety and Security (the Commission) to make recommendations concerning government regulation of the airline industry as it relates to safe air travel in the United States. The Commission recommended a need to decrease the rate of accidents by a factor of five within a decade. They proposed to accomplish this by revising the Federal Aviation Administration's (FAA) regulatory certification program. The Commission led to the establishment of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), which performed some of the first studies regarding aging aircraft and their electrical wire interconnect systems (EWIS) in order to determine the airworthiness of commercial airplanes.
The delivery of electrons throughout the aircraft is such an essential function that wiring can not be regarded as a “fit and forget” item. Detailed inspections of in-service wiring on present commercial airplanes show that EWIS problems are common to both large and small transport aircraft. Moreover, today's jet aircraft rely on more and more sophisticated electrical and computer systems, placing a premium on the reliability of wiring, power feeder cables, connectors and circuit protection devices. Wiring is now recognized as a vital aircraft-level function and is regarded as its own important system on board an airplane.
The physical failure of wiring can cause significant damage to the aircraft EWIS, but can also cause significant collateral damage to other aircraft systems, and potentially lead to an aircraft-level failure. Wires have several modes of failure, each capable of compounding into other problems. Wires can open and they can short to other wires or to ground. These modes of failure are often treated in reliability and risk analyses of electrical systems.
Another failure mode, electrical arc track, is a phenomenon that can cause significant collateral damage to aircraft systems and are not adequately treated in airplane risk assessments. An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. The arc occurs in the gas-filled space between two conductive electrodes (such as the frayed ends of two damaged pieces of wire) and it results in a very high temperature, capable of melting or even vaporizing nearby objects. This phenomenon should be understood in order to adequately assess the potential for collateral damage in such a wire failure event; however this type of wire failure has not been included in traditional risk assessment methods.
It is known that wiring malfunctions have contributed to turn-backs and in-flight diversions, some involving declared emergencies and, in rare but tragic instances, wiring malfunctions have progressed to loss of the aircraft. Furthermore, the amount of wiring in transport category aircraft continues to increase over time, with no plateau yet visible. As the amount of wiring on an airplane increases, so does the potential exposure to wiring failures. Today, most large transport category aircraft contain roughly 200-300 miles of wiring. The amount of wiring on modern aircraft has made the weight of the wiring a non-trivial issue. To compensate for the additional miles of wire aboard an aircraft, manufacturers have been forced to use progressively thinner insulation. Some research indicates that the average amount of insulation provided for each wire has decreased by a factor of seven since the 1950s. As the amount of wire on an airplane increases and the insulation becomes progressively thinner, the potential for failure also increases.
As modern aircraft increase in size and complexity, the addition of advanced navigation, communication, control, and entertainment systems results in an extremely complex network of electrical connections. Because space is at a premium aboard today's aircraft, wires are often bundled together in groups ranging from several wires to several hundred wires. This can force the collocation of wires from several vital aircraft systems, creating a situation where a failure in one set of wires could cause cascading damage that affects several vital aircraft systems and could potentially compromise the overall airworthiness of the airplane. The increase in the size and complexity of an aircraft's EWIS, coupled with weight concerns that necessitate progressively thinner and lighter insulation materials, result in a distinct increase in the potential for EWIS failure. These issues and their consequences necessitate the formal treatment of the risk of the EWIS and its connectivity to the aircraft as a whole.
In order to counteract these growing problems, several sets of goal-oriented laws have passed that attempt to direct manufacturers and operators to achieve the goals in the way they feel is best. Because these goal-oriented laws do not set forth guidelines for achieving the safety goals, the manufacturers and operators must innovate in the way they perform safety analyses.
One such law set by Federal Aviation Regulation, FAR 25.1309: Equipment, Systems and Installations, sets the following two goals:                (1) The occurrence of any failure condition which would prevent the continued safe flight and landing of the airplane is extremely improbable, and        (2) The occurrence of any other failure condition which would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions is improbable.        
FAR25.1309 goes on to state that compliance with these goals must be shown by some form of analysis or test. However, the general language of this regulation has led to confusion as to what is a valid method of showing compliance. Additionally, the complexity of aircraft EWIS, including complicated networks of statistical dependence, non-obvious collocations of vital aircraft systems, and failures in wire insulation that are undetectable by visual inspection make verification of these goals difficult. Assigning a probability of failure to a system that is this complicated is an extremely difficult task that requires significant time, energy, and financial investment.
Due to the complexity and difficulty in demonstrating compliance with FAR 25.1309, the FAA issued a variety of documents intending to demonstrate ways to show compliance with the regulation. Several of these documents, however, proved to be too general and thus provided the manufacturers and operators with little more than initial guidance.
As a result of the continued difficulty in determining ways to comply with FAR 25.1309, the Society of Aircraft Engineers (SAE) published several documents in 1996 directed to providing a uniform method of compliance with the regulation. These documents, such as Aerospace Recommended Practice (ARP) 4754 and ARP 4761, attempt to clarify and outline a number of systematic approaches to showing compliance with FAR 25.1309. These documents, however, still do not provide sufficient guidance for airplane manufactures, operators, and certification authorities to establish a standard method of compliance with FAR 25.1309.
By January 2002, approximately five years after the issuance of ARP4761, the FAA's Airport and Aircraft Safety Research and Development Division (AAR-400) determined that accidents and incidents that have occurred in the past indicate that the assessment tools used in the development and assessment of Electrical Wire Interconnect Systems remained insufficient. In February of 2002, the FAA determined that in modern civil aircraft, EWIS failures are “latent with no official certification check requirement available.” In addition, the FAA concluded that the more complex systems of current generation aircraft design necessitate more sophisticated analysis techniques.
Although the latest version of FAR1309 issued in September of 1977, the complexity of airplane electrical wire interconnect systems has increased steadily in the past 30 years. Therefore, compliance with FAR1309 has become increasingly more difficult to demonstrate. Analysis techniques and limits in computational power have not improved or increased as quickly as the technology found on modern airplanes. Evidenced by the evolution of regulatory structure and procedures, the speed at which airplanes evolve and increase in complexity has left aircraft manufacturers and operators ill-equipped to evaluate the reliability of their systems in any sort of quantitatively rigorous fashion. Previous efforts to show compliance with federal regulations have fallen short of identifying all potential failure modes and fail to address the cascading failure of multiple independent functions within wire bundles and consequential damage and failure of subsystems located adjacent to or in the vicinity of EWIS failures. The problem of meeting the federal guidelines remains.
Risk assessment in the aircraft industry as performed on a modern airplane consists of both qualitative and quantitative components. Risk assessments are not designed to assign a single level of risk to an entire aircraft. Rather, different aspects of a risk assessment are used to calculate failure probabilities, event probabilities, aircraft functional reliability, collateral damage calculations, and many other components of risk. Each of these aspects may be used in assessing both quantitative and qualitative risk levels that are deemed acceptable by the aircraft industry and government agencies.